Consumer demands for smaller products, at lower cost with faster operation and more features, are placing demands on the size and density of printed circuits. One of the most challenging tasks for the manufacturer of circuit boards is to form electrical connections to contact a series of drilled or individually punched holes.
Mechanically formed holes contribute large-tolerance errors to the precision of the overall circuit board, which reduces the potential for increased wiring density. In addition, the production costs of circuit boards are also affected by the use of finer drills which provide more room for circuitry but have shorter lives, and tend to wander. A costly by-product of alignment tolerance is the requirement for drilling smaller stacks of boards on the drilling machine as a result of drill wander. Drilling is already a slow and costly operation, and restriction to drilling fewer circuit boards simultaneously contributes additional costs and production delays.
A common practice for registration on existing machines is to use two CCD cameras to align the circuit board panel with master image films by observing two drilled holes across the width of the circuit board panel. As illustrated in FIG. 1, a typical circuit board panel includes two holes spaced apart along a horizontal, or X axis, for use in alignment. The registration of the circuit board panel to a master film is typically done with a dot on the film to be centered within the perimeter of a hole in the circuit board panel, as illustrated in FIG. 2.
In these conventional schemes, it can be assumed that drilling tolerance is of the order of +/-20 microns, although, depending upon number of factors, mechanical drilling may only be accomplished within a tolerance of +/-75 microns, with predominantly random factors contributing to positional errors. FIG. 3 displays a 100-hole Gaussian simulation of errors encountered in the hole location in one axis, as simulated using a random number generator. The distribution probability of the error of the ultimate hole location along a given axis is displayed in the form of a bell curve, as illustrated in FIG. 4. In the X-axis direction, the hole locations are averaged over the two spaced holes, with resultant registration as illustrated in FIG. 5.
When these hole locations are used to position a circuit board panel relative to an image on a master film, the drill error is reflected in misalignment of panel and film. In addition, there is a high probability that the two positioning holes in the circuit board panel will typically and frequently occupy positions with errors that are displaced in opposite directions from one another. This induces a rotational error in the panel that can increase the positional error over the drilling error, for example, by 12% on a 610.times.710 mm panel, as illustrated in FIG. 6.
Alternative conventional schemes use four targets to position the panel relative to the master film, as illustrated in FIG. 7, by using moving cameras or multiple cameras. This provides the stability of a two-point sample in both X and Y axes and reduces the rotational error, but increases cost and reduces production rate.